1. Field of the Invention
The subject matter of the invention is a pipette with a pipette shaft which has been produced preferably from thermoplastic onto which a likewise plastic pipette tip is slipped. More specifically, the invention relates to a pipette which has an ejector sleeve which surrounds the pipette shaft and an ejector actuator, the pipette shaft being made for slipping on a likewise plastic pipette tip which has a preferably slightly conically opening slip-on section, the pipette also being provided with an attachment section which is formed on the lower end of the pipette shaft and onto which the slip-on section of the pipette tip is slipped. A sealing section is provided on the attachment section which is formed of an elastically-flexible plastic material which seals well with low withdrawal forces. The ejector sleeve is pushed down axially relative to the pipette shaft by the ejector actuator in order to eject the pipette tip which has been slipped onto the pipette shaft after completing the pipetting process or several pipetting processes,
The subject matter of the invention is also a process for producing such a pipette by which the sealing section of elastically-flexible plastic material is directly retentively molded onto the plastic of the pipette shaft.
2. Description of Related Art
Both individual pipettes and also multichannel pipettes or larger pipetting devices are subject to the problem that the pipette tip must be slipped on the pipette shaft, forming a seal but, at the same time, the tip must be able to be pushed off from the pipette shaft by means of mechanical ejection with as little expenditure of force as possible. High withdrawal forces of the pipette tip require a high expenditure of force when the ejector sleeve is actuated. Therefore, providing transmission devices between the ejector actuation and the ejector sleeve is already known; but, this increases the actuation paths.
The problem of withdrawal forces is especially great in pipettes known to date in which the pipette shaft is produced from a relatively hard, wear-resistant plastic and the tip is produced from somewhat less hard, deformable plastic, especially polypropylene. The attachment section of pipette shaft can be easily damaged. The conically matched surfaces then become loose. The user tries to compensate for this by especially vigorous pushing of the pipette tip onto the attachment section of the pipette shaft.
In the prior art, the attempt has already been made to circumvent the existing, aforementioned problem by producing an actively deformable seal between the slip-on section of the pipette tip and the attachment section (International Patent Application Publication WO 91/16975). To do this, an O-ring or some other elastically deformable material is used. However, here, there is the problem of cleaning and the structurally comparatively great effort for the actuating mechanism. Moreover, the existence of gaps is a problem. Overpipetted sample liquid can be carried over and a following specimen can be contaminated.
Nevertheless, the above explained concept has been developed differently. The pipette which is known in this respect and which forms the starting point for the teaching of this patent application (U.S. Pat. No. 4,863,695) has a separate sealing cone of effectively sealing, elastically-flexible plastic material which is slipped onto the lower end of the pipette shaft in a recess and ensures the sealing of the attachment section to the slip-on section. Axially above this sealing section which is formed in a separate molding, there is a fixing section which is formed on the material of the pipette shaft itself with a conical contour which corresponds to the conical contour of the slip-on section of the pipette tip which has been slipped on there at the top. On the pipette shaft, there is a peripheral ring flange as the stop for pushing on the pipette tip.
The above explained pipette known from the prior art has the advantage that this construction of the pipette shaft enables the use of different pipette tips from different manufacturers. The pipette shaft is relatively insensitive to tolerances both with respect to the pipette tips used and also with respect to the accuracy of the slip-on process. In any case, the separate formation of the sealing section is a problem in the slipped-on sealing element with respect to gap formation, cleaning possibilities and danger of contamination in the same way as in the above explained prior art.
It should be stated that the above-explained pipette underlying the present invention is provided with an ejector sleeve and ejector actuator for the pipette tip. The initially explained problem of withdrawal forces when ejecting the pipette tip is therefore present there. Secure seating and good sealing of the pipette tip on the pipette shaft, on the one hand, and low withdrawal forces when the pipette tip is ejected, on the other hand, are inherently contradictory requirements which are to be brought into agreement in this prior art by the use of a separate sealing cone.
Another problem in using different generic pipette tips of different manufacturers in conjunction with the known ejector sleeve is that the effective axial length of the slip-on section of the pipette tip is different for the different pipette tips. The effective axial length designated in this way is the length with which the slip-on section of the pipette tip sits on the attachment section of the pipette shaft. The effective axial length of the slip-on section therefore determines what axial distance the end of the pipette tip has from the front edge of the ejector sleeve.
In the prior art, it has already been recognized that there are cases in which the pipette ejector sleeve which is actuated by the ejector only just touches the edge of the pipette tip, or in any case, the pipette tip can no longer be effectively ejected. To solve this problem, the use of an annular adapter has been proposed (U.S. Pat. No. 4,965,050). With such an adapter, the distance from the ejector sleeve and the edge of the slip-on section of the pipette tip can be bridged with an exact fit.